This invention relates to a plasma etching apparatus and also to a method of cleaning a plasma etching apparatus.
Plasma etching is widely used to process a variety of substrates, such as semiconductor wafers. The plasma etching process may be a step which forms part of a process sequence. For example, in the semiconductor industry it is well known to remove material from a wafer surface by a sputter etch process prior to a subsequent metal deposition step. The sputter etch process is typically performed using an Argon plasma. The purpose is to ensure a high quality metal/metal interface resulting in a low contact resistance. The sputter etch step is typically conducted in a pre-clean module. FIG. 1 shows an example of the prior art, pre-clean module 10 of this type. The pre-clean module 10 comprises a vacuum chamber 12 which is surrounded by an inductive coil 14. A workpiece (not shown) is supported inside the chamber 12 on a platen 16. The workpiece is introduced into the chamber 12 through a wafer loading slot 18. The platen 16 is in a lowered position (shown by solid lines in FIG. 1) to receive the workpiece which has been introduced through the wafer loading slot 18. The platen 16 is subsequently raised into the position shown by the dotted lines in FIG. 1 prior to commencement of the sputter etch process. The inductive coil 14 is connected to an RF power generator (not shown) through an impedance matching network (not shown) as is well known in the art. The pre-clean module further comprises an RF power generator 20 which is connected to the platen 16 through an associated impedance matching circuit (not shown).
The RF power generator 20 is used to bias the platen 16. The walls of the chamber 12 are made from an electrically insulating material such as quartz or ceramic in order to minimise attenuation of RF power coupled into the chamber 12. In operation, a gas (typically Argon) is introduced into the chamber 12 at a relatively low pressure (typically 1-10 mTorr) through a mass flow controller. RF power coupled via the coil 14 generates a plasma. The bias applied to the platen 16 is then used to accelerate ions from the plasma towards the wafer, where ion bombardment etches the surface of the workpiece.
The workpiece may contain substances which outgas heavily. Examples of such substances are organic passivation layers, adhesives, photoresists and spin-on materials. Additionally, novel substrate materials are increasingly being used that outgas heavily. Within the advanced wafer packaging industry, these materials can include polyimide (PI) and polybenzoxazole (PBO) which are especially problematic. When these materials outgas, contaminants are released into the process tool which raises the pressure in the chamber. This can cause significant problems in recovering acceptable vacuum levels in the chamber between the processing of successive workpieces.
There is an additional problem associated with prolonged or frequent etching of polymeric materials on a workpiece. This is the build up of carbonaceous material around the lid and walls of the chamber. The carbonaceous material can accumulate to a point where particulate material becomes loose. There is then a risk that the loose particulate material can fall onto the workpiece causing contamination. Additionally, these particulate materials release gaseous by-products which increase the base pressure of the chamber. This results in poor vacuum levels, and also results in associated problems with process control.
Another problem still is that the etching of any conductive layers can lead to a build up of conductive material on the chamber wall. This gives rise to a conductive coating which can attenuate RF power coupled into the chamber by the coil. As the conductive coating builds in thickness over time, the attenuation can increase to a point where the etch process is affected. For example, problems such as etch rate drift, etching uniformity, and problems with igniting or sustaining the plasma may occur.
The conventional solution to these problems is to perform a maintenance procedure on a frequent basis. This ultimately results in the chamber being vented to atmosphere so that a mechanical clean can be carried out. This intervention has a significant cost and tool downtime. This is not desirable in a production environment. Instead, it would be desirable to reduce the frequency of these maintenance interventions, to reduce cost whilst improving the uptime of the tool.